Known clamping apparatuses of this type include an apparatus described in Patent Literature 1 (Japanese Unexamined Patent Publication No. 36314/2010 (Tokukai 2010-36314)). The above known apparatus is structured as follows.
Four locking members (grippers) to be inserted into a hole of a to-be-clamped object are arranged circumferentially into an annular shape. Wedge surfaces provided at an upper portion of a clamp rod are respectively engaged, from above, with inclined surfaces provided on inner peripheries of upper half portions of the locking members. Further, inner peripheral surfaces of lower half portions of the locking members are in contact with an outer peripheral surface of a lower portion of the clamp rod.
For clamping, while the locking members are held at an upper position with a predetermined force, the clamp rod is driven downward. Then, the locking members first move radially outward, to come into close contact with an inner peripheral surface of the hole. Subsequently, the to-be-clamped object is lowered through the locking members and the inner peripheral surface of the hole which are in a close contact state, so that the to-be-clamped object is clamped on a seating surface.